In an attempt to increase the efficiencies and performance of contemporary jet engines, and gas turbine engines generally, engineers have progressively pushed the engine environment to more extreme operating conditions. The harsh operating conditions of high temperature and pressure that are now frequently projected place increased demands on engine components and materials. Indeed the gradual change in engine design has come about in part due to the increased strength and durability of new materials that can withstand the operating conditions present in the modern gas turbine engine.
The compressor stage of the gas turbine engine is one area that has seen increased demands placed on it. For example, increasing performance and reliability demands for gas turbine engines require both high compression ratios and reduced compression stages. Relatively higher compression ratios in turn result in high compressor discharge temperatures. A reduced number of compression stages to accomplish higher compression ratios results in higher compressor stage tip speeds and higher bore stresses. These combined demands have made it very difficult to utilize monolithic titanium alloy impellers for HPC stages of gas turbine engines. It would thus be desirable to develop a high pressure impeller that can withstand the increased pressures and temperatures associated with gas turbine engines. It is also desired that the impeller design be suitable to relatively smaller gas turbine engines. It has therefore been conceived that a dual alloy approach, combining a higher strength bore alloy and a high temperature outer blade ring material, offers a viable solution.
A rotary compressor such as an impeller undergoes differing stresses at differing locations. Typically a central opening or bore defines an axis about which the rotor spins. In the case of an HPC impeller, multiple airfoils extend radially outward from a bore and axially along the length of the bore. Additionally impellers wrap tangentially, from an inducer section near the inner diameter to the exducer near the impeller outer diameter. In operation, an impeller receives a fluid, such as air, at an upstream axial position. Due to the rotational movement of the impeller, the air is compressed. Typically a given volume of air that is being compressed is passed from an upstream position to a downstream position in the impeller. As the air exits the impeller, at an outwardly radial position, it is at a relatively higher pressure and temperature than it was when the air first contacted the impeller.
It should be noted that this general structure of a gas turbine impeller is also true of other rotary devices such as turbines found in turbochargers and turbopumps. The principles of the invention described herein are thus applicable to these devices as well.
As mentioned, an impeller is characterized by differing stresses at different impeller locations. Stresses due to rotation are greatest in the bore section. These stresses arise as a result of the high centrifugal forces that develop during high RPM operation. It is this area where cracks tend to develop and propagate. Hence, it is an important design criteria that materials in this area of the impeller have relatively high strength characteristics.
Differences in temperature also occur at different points in an operating impeller. As previously noted, air enters an individual impeller at a relatively lower temperature and pressure. When this same air exits the impeller it is at a relatively higher temperature and pressure. Thus, the upstream leading edge of an impeller airfoil at the inducer experiences relatively lower temperatures; and the outer radial edge of an impeller, the area where compressed gas exits, the exducer, experiences relatively higher temperatures. As a consequence, materials used in the gas exiting region must be selected to withstand these temperatures.
Hence there is a need for an improved impeller design and method to manufacture the same. The improved design should take advantage of material characteristics that provide high strength and high temperature performance. It is desired that the impeller, and the method of manufacturing the impeller, provide improved strength performance in bore regions while also providing improved high temperature performance in the outward radial positions. There is a need that the improved impeller design maintain advantageous weight performance of materials such as titanium alloys. The present invention addresses one or more of these needs.